1. The Field of the Invention
The present invention relates to prosthetic structures and corresponding surgical methods used to relieve pain caused by disorders of the knee joint. More particularly, the invention relates to prostheses and methods for total knee arthroplasty.
2. The Relevant Technology
The knee is the largest and one of the most complicated joints of the human body. The human knee joint serves an essential function to allow individuals to lead a normal life, while performing its function, in many ways, much better than any device heretofore designed by human engineers. For example, the knee should be able to move from 0xc2x0 in the straight position to more than 90xc2x0, while being completely stable in every other direction.
The bones of the knee joint, when functioning properly, move together with very little friction. To function properly, a healthy knee joint requires an intact layer of hyaline cartilage, the material that makes up the articular cartilage on opposing surfaces of the joint. In addition, the bones of the joint must be in proper alignment and the synovial membranes must produce sufficient amounts of lubricating (synovial) fluid. Furthermore, the surrounding ligaments and tendons must prevent the bones from being placed in abnormal positions.
FIG. 1 is an anterior (that is, taken from the front of the body) cross-sectional view of a human knee 10. The knee 10 consists of three bones; a femur 20, a tibia 30, and a patella 40. Each of these bones is covered with articular cartilage, which has a smooth glistening surface. Located at the distal end of femur 20 are the femoral condyles 22 having a medial condyle 24 and a lateral condyle 26 separated by an intercondylar fossa or notch 28. Formation of intercondylar notch 28 is such that patella 40 articulates therethrough during extension and flexion of knee 10.
Tibia 30 supports most of the weight transmitted between femur 20 and the foot (not shown), while a small portion of the weight is carried by a fibula 42 located substantially parallel to tibia 30. As such, tibia 30 has a tibial plateau 32 with a medial plateau 34 and a lateral plateau 36 substantially aligned to cooperate with medial condyle 24 and lateral condyle 26 of femur 20. Medial plateau 34 and lateral plateau 36 of tibia 30 are separated by an intercondylar area formed with an elevated portion or an intercondylar eminence 38. Intercondylar eminence 38 locates within intercondylar notch 28 to maintain structural support between femur 20 and tibia 30. Additionally, intercondylar notch 28 separates the spaces of a medial compartment 44 and a lateral compartment 46 formed between the respective medial and lateral plateaus 34, 36.
Located within each medial and lateral compartment 44 and 46 are the menisci 50, shown by the dotted line. Menisci 50 consists of two crescentic lamellae formed to distribute surface stresses between femur 20 and tibia 30. As such, the upper surfaces of the menisci 50 are smooth and concave to accommodate femoral condyles 24 and 26, while the lower surfaces are smooth and flat to cooperate with tibial plateaus 34 and 36.
Surrounding and stabilizing the dynamic structure of knee 10 are a number of varyingly sized ligaments. Particularly, four main ligaments maintain the stability and flexibility of knee 10: a medial collateral ligament 62, a lateral collateral ligament 64, an anterior cruciate ligament 66 and a posterior cruciate ligament 68. Medial collateral ligament 62 and lateral collateral ligament 64 limit side-to-side motion. Medial collateral ligament 62 extends from medial condyle 24 or portions of femur 20 to medial plateau 34 of tibia 30. Similarly, lateral collateral ligament 64 extends from lateral plateau 36 or portions of femur 20 to fibula 42. Anterior cruciate ligament 66 and posterior cruciate ligament 68, so named because they cross in the middle of knee 10, are rope-like ligaments formed from interwoven and overlapping fibers. Anterior cruciate ligament 66 and posterior cruciate ligament 68, extend from the anterior to the posterior of knee 10 and prevent femur 20 and tibia 30 from sliding forward and backward while permitting a wide range of rotational movement.
While the knee generally serves its purpose very well, various disorders of the knee cause a great deal of pain and loss of mobility and function to those who are affected with such disorder. Some knee disorders are congenital; that is, they are present at birth. Other disorders of the knee are brought on by bacterial infections that may occur at any age. Yet still other disorders result from normal xe2x80x9cwear and tearxe2x80x9d of the knee joint, whether such xe2x80x9cwear and tearxe2x80x9d arises from age or injury. Perhaps the most wide spread disorder of the knee is arthritis. The term xe2x80x9carthritisxe2x80x9d is generally used as a common name for the effects of several degenerative knee disorders, such as by way of example traumatic arthritis, infectious arthritis, osteoarthritis, and rheumatoid arthritis.
Of various types of arthritis, osteoarthritis is perhaps the most common. Osteoarthritis is a degenerative xe2x80x9cwear and tearxe2x80x9d process that affects substantial numbers of people. The final result of unchecked osteoarthritis is damaged articular cartilage, and subchondral bone which in many cases causes extreme pain as the damaged surfaces are rubbed together during joint movement. Osteoarthritis may also be caused by angular deformity or old fractures. Systemic arthritis such as rheumatoid arthritis or gout affects the synovium (the membrane tissue in the knee that normally lubricates the knee), becomes pathologic and one or more surfaces of the joint are destroyed.
Osteoarthritis may also involve the development of abnormal bone subjacent to the joint surface, known as subchrondral lesions. These subchrondral lesions may take the form of a cyst or sclerosis. Due to the decreased stability of the knee through the generation of cysts and sclerosis and decreased joint space, marginal spurs develop in an attempt to stabilize the joint. Unfortunately, the spurs also cause severe pain, stiffness decreased range of motion, loss of stability, and loss of function.
Generally, osteoarthritis affects people past the age of 60 years without providing any easily recognizable single cause. However, osteoarthritis may develop in younger people due to congenital disease. Furthermore, traumatic injury may cause development of an osteoarthritis condition, such as from various sporting activities.
In the prior art, several methods have been used for alleviating the pain and improving the function of a knee joint affected with a degenerative disorder such as osteoarthritis. One of the most common procedures used in treatment of knee disorders is know as xe2x80x9carthroplastyxe2x80x9d and entails the implantation of an artificial joint component into the knee. Arthroplasty has been one of the major areas of advancement in knee surgery during the past quarter century. Knee arthroplasty can take the form of unicompartmental arthroplasty or total knee arthroplasty.
Unicompartmental arthroplasty involves replacement of one of the two compartments of the knee joint. For example, this procedure is used where either of the medial or lateral compartments is damaged, while the remaining compartment and intercondylar notch are otherwise normal. In such a case, it is beneficial to replace the damaged areas of the femoral condyle and tibial plateau with an artificial prosthesis that will work in conjunction with the natural portions of the knee.
The most common arthroplasty procedure used to alleviate pain and restore knee function is total knee arthroplasty, also called total knee replacement. While many different styles of total knee replacement prostheses have been implanted in patients, they generally resemble the prosthetic illustrated in FIG. 3.
Conventional total knee replacement involves a complete resurfacing of both tibial plateaus 34, 46 and femoral condyle 24, 26 as suggested in FIGS. 2 and 3. The conventional surgical procedure used during total knee replacement involves the insertion of one or all of the following artificial components into the knee; a one piece metallic femoral component 70, a one piece metallic tibial tray component 74 with a polyethylene insert (not shown), and a one piece patellar component (not shown) of polyethylene. Each component 70 and 74 is adapted to cooperate one with another. As commonly used, femoral component 72 and tibial component 74 include one or more stems 78, as shown in FIG. 3, that are used to fixably attach respective components 72 and 74 to their respective bones.
As depicted in FIGS. 2 and 3, the present procedure for total knee replacement is extremely invasive, causing significant damage to the muscles, tendons and ligaments surrounding knee 10. The traditional surgical procedure entails making a large surgical incision over the front of knee 10. Patella 40 is slipped across to the outside of knee 10 to expose the joint between femur 20 and tibia 30, as shown by the dotted line. Any excess bone, such as subchrondral lesions, formed on either femoral condyles 24 and 26, or tibial plateaus 34 and 36 is removed and tight soft tissue carefully released so that knee 10 returns to its normal shape without becoming too loose. Following removal of excess bone, the worn bone surfaces are cut away, while anterior cruciate ligament 66 and posterior cruciate ligament 68 are removed. The resultant bone surfaces are sized and holes 80 and 82 are drilled into femoral condyle 22 and tibial plateau 24. Specifically, as shown in FIG. 2, hole 80 is drilled into the medullary canal (not shown) to accommodate tibial tray component 34.
During drilling of holes 80 and 82 high tolerances must be maintained, since misalignment of femoral component 70 and tibial tray component 74 results in a misaligned knee joint, thereby eliminating the beneficial effects of the surgical procedure. Once prepared, exact replicas of the real artificial components are placed in position with holes 80 and 82 to allow testing of joint stability and dynamic motion. Upon completion of testing, the replicas are removed and the prosthetic components are fixed in position.
The most commonly accepted method of fixing femoral and tibial tray components 70 and 74 to femur 20 and tibia 30 is through the use of a cement, such as polymethyl methacrylate (PMMA) or porous ingrowth press fit. PMMA is a two-component acrylic cement that has the advantage of accepting a rapid setting time. After mixing the two components of the acrylic cement, holes 80 and 82 are xe2x80x9cpackedxe2x80x9d with unset PMMA. The stems of femoral component 70 are located within holes 82 while stem 78 of tibial component 74 is located within hole 80. Both femoral component 70 and tibial component 74 are maintained in place until PMMA sets. Once the PMMA has set, the insert (not shown) is located between tibial tray component 74 and femoral component 70 and the necessary positioning of patellar component 72 is performed.
Due to the various sizes and dimensions of knee 10, each component 70 and 74 is available in a wide variety of sizes and styles. During the surgical procedure, 10-20 different prosthetic components may be available to a physician. Currently, most of the prosthetic femoral components 70 and tibial tray components 74 are fabricated from alloys containing stainless steel, chromium, cobalt, molybdenum, or titanium. Such materials are inert within the body and maintain good mechanical properties. Tibial tray component 74 is most often made of titanium or stainless steel that is strong and leaves space for the insert. The insert traditionally comprises a plastic material, usually manufactured from an ultra-high molecular weight polyethylene (UHMWP), because it is chemically similar to ordinary polyethylene but much harder and very smooth. Unfortunately, there is a significant manufacturing cost with these prostheses.
While the conventional total knee prosthesis procedure has been popularly accepted, major risks and drawbacks accompany its use. First, the required long incision either anteriorly or posteriorly disrupts the extensor mechanism of the knee, such as the quadricep muscles, thus prolonging rehabilitation.
Second, traditional total knee replacement requires the removal of a large amount of bone from femur 20 and tibia 30, while necessitating insertion of a large amount of foreign material. Insertion of foreign material creates a significant xe2x80x9cdead spacexe2x80x9d in the knee where an individual has no feeling, while increasing the risk of infection.
Third, the total knee replacement procedure eliminates the stability provided by both anterior and posterior cruciate ligaments 66 and 68 of the knee 10. During current surgical procedures, commonly both anterior and posterior cruciate ligaments 66 and 68 are removed. Therefore, only the combination of medial and lateral collateral ligaments 62 and 64 with the configuration of the prosthesis maintains a stability of knee 10.
Fourth, since tibial tray component 74 covers both the medial and lateral portions of tibial plateau 32, a tilting motion occurs during normal motion of knee 10. The tilting motion causes a predisposition to loosening of the bond between tibial tray component 74 and tibia 30. Furthermore, the abnormal motion of medial and lateral portions of tibial plateau 32 result in abnormal dynamic knee motion, thereby disrupting the normal xe2x80x9cscrew mechanismxe2x80x9d of the knee, i.e. the external rotation of tibia 30 relative to femur 20 in the final 20xc2x0 motion of the knee extension. As such, an abnormal gait pattern can occur resulting in a predisposition for serious complications (such as loosening, infection, osteolysis, bone loss, etc).
Fifth, as mentioned earlier, the most common method of fixing the components of a total knee prosthesis is by way of PMMA. PMMA cement is prepared by mixing two components together which harden into a solid mass by way of a chemical process. One of the two components is a fine granular powder of prepolymerized polymethyl methacrylate and the other component is a liquid monomer. One constituent of the liquid monomer is N, N-Dimethyl-Para-Toluidine (DMPT), a toxic material. Other monomer ingredients also exhibit adverse effects on humans. Thus, the introduction of the mixed, but yet unset, PMMA, cement mixture into holes 80 and 82, presents the potential of introducing a significant amount of toxic material into the blood stream, especially when locating the stems of the prosthesis into the medullary canal. Various reactions can occur to PMMA cement, such as hypotension and even circulatory system collapse.
Aside from the immediate hazards that attend the use of PMMA cement, concern has also been expressed that there may be long term toxicity, hypersensitivity, and carcinogenicity resulting from the materials that make up prior total knee prosthesis, including cobalt, chrome, titanium, and polyethylene. In view of the uncertainty of the effects of long-term use of these materials within the human body, it has been considered advisable to reduce the contact between these materials and the body as much as possible.
Finally, in any surgical procedure there is the potential that infection may occur due to entry of microorganisms into the surgical wound. Devastating infections are particularly difficult to prevent in total knee replacement procedures due to the extensive invasion of the body that is required. Special surgical techniques have been developed which reduce the risk of infection to the patient; unfortunately, these surgical techniques require far greater care than other types of surgical procedures, and in some cases are extremely cumbersome.
It is a primary object of the invention to provide structures and methods for reducing pain and instability, while improving function to damaged or diseased knees.
A particular object of the present invention is to provide structures and methods for augmenting portions of the femur and the tibia, in order to reduce or eliminate pain, improve range of motion, and enhance function of the joint.
Another object of the invention is to provide structures and methods that minimize the quantity of bone mass required to be removed for total or unicompartmental knee arthroplasty.
Another object of the invention is to provide structures and methods for repairing a damaged or diseased knee joint which are significantly less invasive than prior art methods of knee arthroplasty.
Yet another object of the invention is to provide structures and methods for reducing pain and increasing function of a diseased knee joint in which foreign material presented to the body tissues and fluids is reduced to a minimum.
Still yet another object of the invention is to provide structures and methods for repairing damaged or diseased portions of a knee that minimize the possibility and consequences of infection entering the wound resulting from knee arthroplasty.
Another object of the invention is to provide knee prosthetic structures that are simpler to manufacture and cost less than those prostheses available at present.
Yet another object of the invention is to provide structures and methods that reduce the wound size, surgical procedure time, and length of accompanying in hospital stay.
Still another object of the invention is to provide structures and methods for reducing pain and increasing function of a knee that results in a less painful rehabilitation while achieving increased range of motion with similar stability as that of a healthy knee joint.
Still yet another object of the present invention is to preserve the anterior cruciate ligament and the posterior cruciate ligament to preserve a more normal gait pattern.
Yet another object of the present invention is to preserve the normal radius of curvature of the knee, thus preserving the screw home mechanism of the knee.
Another object of the present invention is to provide for more efficient boundary lubrication of the prosthetic components, thus reducing friction and associated problems.
To achieve the forgoing objects and in accordance with the invention as embodied and broadly described herein, a knee joint prosthesis is disclosed. The knee joint prosthesis includes one or more femoral components and one or more tibial components. The femoral components are adapted for attachment to a femur of a patient while retaining the cruciate ligaments. Each femoral component has a generally spheroidal weight-bearing surface and includes a stem that is used to fixably attach the femoral component to the patient""s femur. The tibial component has a first surface configured to aid in attachment of the tibial component to a tibia while a second surface is adapted to cooperate with the bearing surfaces of the femoral components.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.